Reactivated Paleokarst in Northern Audrain County, Missouri

Abstract

Three sinkholes opening in Audrain County, Missouri in 2008 offered a rare opportunity to examine karst features in an environment in which they rarely occur. The regional geology around the site consists of Mississippian-age limestone, overlain by thick Pennsylvanian-age cyclothems of shale, sandstone, limestone and coal, covered with thick glacial tills from multiple Neogene-age glacial episodes. Site investigations revealed that the sinkholes contained intact lithified sediments deposited at the end of the Mississippian. This paleofill material has rarely been found in other sinkholes within Missouri. The paleofill demonstrates that karst in this region of Missouri is potentially a phenomenon formed during an episode of regression. During this regression, subaerial exposure allowed karst processes to form the sinkholes, or paleosinks, which were subsequently filled with colluvial materials. These materials, consisting of sand, clay and chert derived from nearby siliciclastic sources south of the site, were deposited in the open sinkhole after Mississippian time. The sediments were buried deeply enough during early Pennsylvanian time to weakly alter the sediments, allowing diagenesis to compact clay into shale and cement sand into sandstone. Chert clasts were also incorporated into these deposits. Carbonaceous deposits at the base of the sediments indicate organic material deposition. These materials could have provided sulfur for pyrite deposits observed in fossil molds in the chert-rich sediment. The paleosinks indicate the possible existence of a more extensive, contiguous zone of karst that could have existed in northern Missouri, eastern Iowa, and into southern Wisconsin. Observations around the site may indicate a multi-story karst aquifer could have existed in the area before the beginning of post-Cretaceous glacial cycles. Similar paleokarst sites may also lie hidden near areas where active stream downcutting has removed the glacial and Pennsylvanian overburden, and could be hydrologically reactivating other quiescent sinkholes in the area around the site.